1. Field of the Invention
This invention relates to a drill which enables performance of smooth and stable drilling even under severe conditions, specifically, such as high speed and dry drilling.
2. Background Art
A drill designed to be suitable for use under severe drilling conditions in which no or small amounts of cutting lubricant is used is disclosed, for example, in Japanese Unexamined Patent Application, First Publication, No. 2000-198011. The drill disclosed in the above Publication comprises: a center cutting edge extending from a chisel at the tip of a drill main body; a major cutting edge extending outward from the end of the center cutting edge; an outer corner cutting edge obliquely extending in the direction opposite to the direction of drill rotation from the end of the major cutting edge at a certain radius of rotation; and a leading edge consisting of a chip discharge flute and a margin portion, wherein a straight or curved chamfer is formed on the leading edge so as to be connected to the corner edge. The cross angle between the outer corner edge and the margin portion at the periphery of the drill main body is set as an obtuse angle so that the cutting edge at the periphery is prevented from being broken even under severe drilling conditions.
A drill in which a portion of the cutting edge near the periphery is slightly inclined in the direction opposite to the direction of drill rotation as in the above drill is disclosed in Japanese Examined Patent Application, Second Publication, No. Hei 4 -46690. In this drill, a primary straight ridge and a secondary straight ridge located on the outer portion of the cutting edge form a convex shape, i.e., an approximate V-shape, when viewed in the axis of rotation. In addition, when viewed in the axis of rotation, the inner portion of the cutting edge is also substantially formed in a V-shape, which consists of a straight chisel edge formed by thinning and a straight connection ridge connected to the straight chisel edge; furthermore, a portion connecting the secondary straight ridge to a inner ridge is formed as a rounded concave.
A drill in which the cutting edge is formed in a concave shape, when viewed in the axis of rotation, as in the above drill is disclosed in Japanese Unexamined Patent Application, First Publication, No. Sho 61-58246, etc. In this drill, a radial rake angle of the cutting edge at the periphery is set to zero or a positive value.
As disclosed in Japanese Patent No. 2674124, a drill comprising: a drill main body rotatable about an axis of rotation, and having a tip end with a tip flank and a shank; a chip discharge flute formed on a periphery of the drill main body, and extending from the tip end toward the shank of the drill main body; and a cutting edge formed along an intersecting ridge where a wall surface of the chip discharge flute facing in the direction of drill rotation intersects the tip flank of the drill main body, is known.
A drill such as disclosed in Japanese Unexamined Patent Application, First Publication, No. Sho 61-58246, in which the outer portion of the cutting edge is formed in a rounded concave shape, enables performance of smooth and stable drilling, under normal drilling conditions, in which curled chips are smoothly discharged; however, under severe drilling conditions, the service life of the drill tends to be substantially shortened due to breakage or chipping of the cutting edge at the periphery due to lack of strength of the drill main body since the cross angle between the cutting edge at the periphery and the margin portion is an acute angle.
On the other hand, in such a drill in which the outer portion of the cutting edge is formed in a convex shape, i.e., in a V-shape, when viewed in the axis of rotation, and the cutting edge near the periphery is slightly inclined in the direction opposite to the direction of drill rotation, as disclosed in Japanese Unexamined Patent Application, First Publication, No. 2000-198011 or Japanese Examined Patent Application, Second Publication, No. Hei 4-46690, it is possible to prevent breakage or chipping of the cutting edge by making the cross angle between the cutting edge at the periphery and the margin portion to be an obtuse angle; however, the chips produced by the cutting edge are broken at the tip of the V-shape, and they tend to twine with each other and to clog the chip discharge flute under high speed drilling conditions. In addition, because the chips which are produced by the cutting edge and flow to the chamfer of the wall of the chip discharge flute or to outside the V-shape tend to move out of the drill, the curling performance of the chips are generally degraded and non-curled chips are strongly pressed against the trailing wall surface of the chip discharge flute, which applies a great frictional resistance to the drill main body, which leads to rapid wear of the drill and to increase in a driving torque applied to the drill for drilling.
When a thinning cutting edge such as a chisel edge connected to the chisel or a center cutting edge is formed in the inner portion of the cutting edge by applying thinning, the chips produced by the thinning cutting edge flow along the bottom surface of the thinning portion extending from the wall of the chip discharge flute to the tip end and which is formed by thinning. In a drill such as disclosed in Japanese Examined Patent Application, Second Publication, No. Hei 4-46690, in which the thinning portion is formed by simply cutting the end portion of the wall of the chip discharge flute to define a V-shaped recess, the chips produced by the thinning cutting edge tend to clog at the bottom of the V-shaped recess, and even to cause fusion since the chips are clogged and not cooled, specifically under high speed and dry drilling conditions. The bottom of the V-shaped recess of the thinning portion obliquely extends from the wall of the chip discharge flute toward the inner end of the cutting edge. When the oblique angle of the bottom is small, the bite performance of the drill tip on a workpiece may be degraded and the driving torque for drilling may be increased due to increase in a thrust force, because the innermost point of the thinning cutting edge, i.e., of the cutting edge is positioned far outside from the center of the tip flank, and thus the chisel becomes wide. On the other hand, when the oblique angle of the bottom is large, the tip angle of the drill main body measured in cross section along the bottom becomes small, which may lead to breakage of the drill tip under high speed and dry drilling conditions.
In a drill such as disclosed in Japanese Unexamined Patent Application, First Publication, No. 2000-198011 or Japanese Patent No. 2674124, the rake angle of the cutting edge, when viewed in cross section perpendicular to the cutting edge, i.e., the orthogonal rake angle of the cutting edge is generally designed so as to gradually increase from the center of the drill main body to the periphery in accordance with the radius of rotation (See, for example, drills E and F referred to below and FIG. 12). In a drill disclosed in Japanese Patent No. 2674124, the cutting edge tends to experience chipping or breakage and rapid flank wear because the rake angle is the largest at the periphery, in contrast, the included angle of the cutting edge is the smallest, which decreases strength of the cutting edge at the periphery where the distance from the axis of rotation is the largest, the cutting speed is the highest, and consequently the cutting condition is the most severe in high speed and dry drilling.
On the other hand, in a drill disclosed in Japanese Unexamined Patent Application, First Publication, No. 2000-198011, in which outer corner cutting edge obliquely extending in the direction opposite to the direction of drill rotation from the end of the major cutting edge is formed, the orthogonal rake angle of the cutting edge is decreased as the radial rake angle of the cutting edge is increased at the outer corner cutting edge, and the orthogonal rake angle is increase in accordance with the radius of rotation; therefore, the orthogonal rake angle must be set to a relatively small value all along the cutting edge in order to ensure a sufficient strength of the cutting edge even under the severe conditions as described above. In this case, the cutting ability is degraded all along the cutting edge, and thus the cutting resistance is increased, which may cause a substantially shortened service life of the drill due to rapid wear under the severe cutting conditions such as high speed and dry cutting, and may even cause a breakage of the drill main body itself because of an excessive torque applied to the drill due to increased frictional resistance.